1. Field of the Invention
The present invention related to a grain-oriented electrical steel sheet, the magnetic properties of which are only slightly deteriorate by stress relief annealing, and to a method and apparatus for producing the same.
2. Description of the Related Arts
It is important in the light of energy saving to lessen the watt loss of grain-oriented electrical steel sheet.
Japanese Examined patent publication No. 58-26405 discloses a watt-loss reduction method, wherein the magnetic domains are subdivided by laser irradiation. The strain induced by the laser irradiation causes a watt loss reduction. The above mentioned method can therefore be applied to the grain-oriented electrical steel sheet for use in the production of laminated cores, which need no stress relief annealing, but cannot be applied to grain-oriented electrical steel sheet for use in the production of wound cores, which need stress relief annealig.
Japanese Unexamined patent publication No. 56-130454 discloses a watt loss reduction method wherein strain is induced to a steel sheet after the secondary recrystallization annealing and the cluster of minute crystal grains, which are formed during the heat treatment after the secondary recyrstallization annealing due to the strain, are utilized for watt loss reduction. Since the cluster of minute crystal grains is formed on the surface of secondary recrystallized steel sheets, the watt loss characteristic is not impaired by the stress relief annealing. It is, however, difficult, by the method of Japanese Unexamined patent publication No. 56-130454, to obtain such a low watt loss as attained by the laser irradiation method.